Women in the studies who took the drospirenone-containing pills, such as Gianvi, Ocella, Yasmin, Yaz, and Zarah, had a twofold to threefold increased risk of nonfatal blood clots compared to women who took oral contraceptives that contained levonorgestrel, such as Jolessa, LoSeasonique, Quasense, Seasonale, and Seasonique.

But the maker of Yasmin and Yaz says the study was flawed.

Study researchers say the risk to individual users is small. By their estimates roughly three women out of 10,000 who take the drospirenone-containing pill for a year could be expected to develop a clot, compared to about 1.2 women in 10,000 who take levonorgestrel.

“The [clot] risk in pregnancy is much higher than for any oral contraceptive,” study researcher Susan S. Jick, PhD, of Boston University School of Medicine, tells WebMD. “Nevertheless, women taking these pills or considering taking them should be aware that there is very likely an increased risk associated with their use.”

At least four other studies have linked use of drospirenone-containing birth control pills to an increased risk for pulmonary embolisms and deep vein thrombosis. But all four studies included women with known risk factors for the conditions.

These women were excluded from the newly published studies, in which investigators examined medical claims data among oral contraceptive users in the U.S. and the U.K.

In the U.S. study, users of the drospirenone-containing pills were twice as likely to experience nonfatal blood clots as users of the older-generation levonorgestrel-containing pills.

In the U.K. study, use of the drospirenone pill was associated with a threefold increase in risk.

The researchers call for a systematic review of all the studies examining blood clot risk associated with the use of birth control pills containing drospirenone.

“In the meantime,” they write, “as no clear evidence exists to show that the use of the drospirenone pill confers benefits above those of other oral contraceptives in preventing pregnancy, treating acne, alleviating premenstrual syndrome, or avoiding weight gain, prescribing lower risk levonorgestrel preparations as the first line choice in women wishing to take an oral contraceptive would seem prudent.”